The concept and validity of EDR

michael_15

So like most of you, I've heard the story about dipping radiators in paint and so forth to determine the EDR of radiators. . . .

Is this really true? It doesn't seem to me that this would arrive at the correct answers. The true answers would depend fairly strongly on the shape of the radiator. For example, taller radiators should have lower BTU/h ratings than shorter radiators of the same EDR.

The air in between the columns (or tubes) of my radiators near the top is always warm (naturally). It's well over 100 degrees. This is because the convecting air picks up enough heat on its journey from the bottom of the radiator to the middle of the radiator to reach this temperature.

However, this means that the BTU-h of the top of the radiator is limited by the heat of the surrounding air. If we say that the radiator's output is 30% radiant and 70% convection, then the 240 BTU (steam, 215*) per square foot, which is 168 BTU of convective BTU, is lowered. If the surrounding air near the top of the radiator is, say, 110 degrees (meaning a 105* difference versus a 145* difference), this convective segment may only be worth 112 BTU or something along those lines (2/3rds the power: these are estimates), meaning the BTU output of the square foot of EDR near the top of the radiator is only 184 BTU. (I didn't reduce the radiant portion of heat loss here.)

This, of course, doesn't even get into the fact that the radiant energy coming out of the middle tubes of the radiator aren't living up to their potential.

This lowered heat loss performance based on shape can be seen, for example, when looking at the published ratings for heat loss from bare steel pipe. A 2-inch steel pipe will loses around 405 BTU per square foot of pipe, but a 4-inch pipe loses only 363 BTU and a 10-inch pipe loses 327 BTU. (All of which are greater than the 240 BTU we use for cast iron radiators).

Anyway, this was just going through my mind as I was sitting on the train today. Thought I'd post it and see what you guys think. . . I hope I made sense.